1. Qingdao Institute of Bioenergy and Bioprocess Technology Chinese Academy of Sciences Qingdao 266101 P. R. China
2. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization The University of Toledo Toledo OH 43606 USA